- preg_replace: documentation ( source)
<?php
$str = '<input type="email" name="email_address" id="email_address" value="{default}" placeholder="user@domain.com">';
echo preg_replace('/<input([a-zA-Z_0-9 ={}"]*)type="email"([a-zA-Z_0-9 ={}"]*)\>/', '<input$1type="email"$2 value="lee@tth.media">', $str);